Dwarf Planet

dwarf planet is a planetary-mass object that does not dominate its region of space (as a true or classical planet does) and is not a satellite. That is, it is in direct orbit of the Sun and is massive enough to be plastic – for its gravity to maintain it in a hydrostatically equilibrious shape (usually a spheroid) – but has not cleared the neighborhood around its orbit of other material.The prototype dwarf planet is Pluto.

Ceres as seen from the Dawn spacecraft. It is the only dwarf planet in the asteroid belt.

Pluto as viewed by New Horizons space probe on July 13, 2015.

Eris and its moon Dysnomia viewed by the Hubble Space Telescope.

Makemake and its moon viewed by the Hubble Space Telescope.

Haumea and its two moons Namaka and Hiʻiaka viewed by the Hubble Space Telescope.

The number of dwarf planets in the Solar System is unknown, as determining whether a potential body is a dwarf planet requires close observation. The half-dozen largest candidates have at least one known moon, allowing determination of their masses. The interest of dwarf planets to planetary geologists is that, being differentiated and perhaps geologically active bodies, they are likely to display planetary geology, an expectation borne out by the 2015 New Horizons mission to Pluto and Dawn mission to Ceres.

The term dwarf planet was coined by planetary scientist Alan Stern as part of a three-way categorization of planetary-mass objects in the Solar System: classical planets (the big eight), dwarf planets and satellite planets. Dwarf planets were thus originally conceived of as a kind of planet, as the name suggests. However, in 2006 the term was adopted by the International Astronomical Union (IAU) as a category of sub-planetary objects, part of a three-way recategorization of bodies orbiting the Sun[2] precipitated by the discovery of Eris, an object farther away from the Sun than Neptune that was more massive than Pluto but still much smaller than the classical planets, after discoveries of a number of other objects that rivaled Pluto in size had forced a reconsideration of what Pluto was, Thus Stern and many other planetary geologists distinguish dwarf planets from classical planets, but since 2006 the IAU and the majority of astronomers have excluded bodies such as Eris and Pluto from the roster of planets altogether. This redefinition of what constitutes a planet has been both praised and criticized.